Abstract
The current study focusses on the phase composition, solidification path, and microstructure evaluation of gravity cast Al-4Mg-0.5Si-xLa aluminum alloy, where x = 0, 0.1, 0.25, 0.5, 0.75, and 1 wt.% La. A computational CalPhaD approach implemented in Thermo-Calc software and scanning electron microscopy technique equipped with electron microprobe analysis (EMPA) was employed to assess its above-mentioned characteristics. The thermodynamic analysis showed that the equilibrium solidification path of La-containing Al-Mg-Si alloys consists of only binary phases LaSi2 and Mg2Si precipitation along with α-Al from the liquid and further solid-state transformation of this mixture into α-Al + Al11La3 + Mg2Si + Al3Mg2 composition. Scheil–Gulliver simulation showed a similar solidification pathway but was accompanied by an increase in the solidification range (from ~55 °C to 210 °C). Furthermore, microstructural observations were congruent with the calculated fraction of phases at 560 °C and related to α-Al + LaSi2 + Mg2Si three-phase region in terms of formation of La-rich phase having both eliminating effect on the eutectic Mg2Si phase. Quantitative EMPA analysis and elemental mapping revealed that the La-rich phase included Al, La, and Si and may be described as Al2LaSi2 phase. This phase shows a visible modifying effect on the eutectic Mg2Si phase, likely due to absorbing on the liquid/solid interface.
Highlights
Many studies showed that micro-alloying with rare earth (RE) elements may have remarkable effects related to refining of the as-cast structure, making them efficient modification agents for aluminum alloys [1,2,3,4,5]
La is ubiquitously used as an addition for achieving favorably fine α-Al [15,16], eutectic silicon [1], Fe-bearing phases [5,13], as well as Mg2 Si phase [17,18], one of the main structural components in 511 type cast aluminum alloys studied in this paper
As can be seen from the magnified section (Figure 1b), the liquidus line goes down slightly along with the area related to the formation of the α-Al phase, likely due to nearness to the eutectic point (4.5 wt.% La, 625 ◦ C) adjoining to the area for LaSi2 primary phase appearance
Summary
Many studies showed that micro-alloying with rare earth (RE) elements may have remarkable effects related to refining of the as-cast structure, making them efficient modification agents for aluminum alloys [1,2,3,4,5]. Metals 2020, 10, 1673 binary Al-La and Al-Ce systems [7], these elements both provide a formation of eutectic reactions, L→α-Al + Al11 La3 (11.7 wt.% La, 640 ◦ C) and L→α-Al + Al11 Ce3 (12.2 wt.% Ce, 621 ◦ C) In this respect, several previous studies related to the design of alternative cast alloys for high-temperature applications are published [8,9,10]. La is ubiquitously used as an addition for achieving favorably fine α-Al [15,16], eutectic silicon [1], Fe-bearing phases [5,13], as well as Mg2 Si phase [17,18], one of the main structural components in 511 type cast aluminum alloys studied in this paper
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